<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
     <input type="text" placeholder="请输入一个年份" name="" id="inp">
     <button id="but" >计算</button>
</body>
</html>
<script>
      var inp = document.getElementById('inp')
      var but = document.getElementById('but')
      but.onclick = function(){
          if(inp.value % 4 == 0){
              alert('是闰年')
          }else{
              alert('不是闰年')
      }
    }
</script>